1. Field of the Invention
The present invention relates to a four-wheel steering vehicle in which not only front wheels but also rear wheels are steered in accordance with an operation of a steering wheel, specifically to a four-wheel steering vehicle provided with an anti-lock braking system for controlling a braking force in a manner that the front and rear wheels are kept from locking in a braking operation.
2. Description of the Prior Art
Conventionally, there has been known a four-wheel steering vehicle for a motor vehicle in which both front and rear wheels are steered in accordance with an operation of a steering wheel. Japanese Patent Publication No. 60-44185 discloses a four-wheel steering system in which a steering ratio of a steering angle of the rear wheel to that of the front wheel changes based on a steering ratio property in accordance with a vehicle speed. In the Japanese patent publication, the steering ratio property is basically set to produce a reverse phase in which the rear wheel is steered in a direction opposite to that of the front wheel in a relatively low vehicle speed and produce a same phase in which the rear wheel is steered in the same direction as the front wheel in a relatively high speed of the vehicle so that a turning performance can be improved in the relatively low speed of the vehicle and a driving stability can be maintained in the relatively high speed of the vehicle.
Meanwhile, there has been proposed a vehicle provided with an anti-lock braking system in which a braking force is controlled to prevent the wheels from locking and skidding to thereby obtain a driving stability during a braking operation. Japanese Patent Public Disclosure No. 61-102363 discloses an anti-lock braking system including a braking control device for controlling a hydraulic pressure of a wheel cylinder which applies a braking force to the wheel, a speed detecting device for detecting a wheel rotation speed, and a controller for calculating a slip ratio between the wheel and road surface based on a signal from the detecting device and providing the braking control device with a control signal in accordance with the slip ratio so as to prevent the wheel from locking.
In the anti-lock braking system as disclosed in the Japanese Patent Public Disclosure No. 61-102363, there are usually provided a fail-safe system which switches the braking control from an anti-lock braking control in which the braking force is controlled based on the slip ratio to a normal braking control in which the braking force is applied to the wheel cylinder in accordance with the braking operation for a brake pedal regardless of the slip ratio when an abnormal condition occurs in the anti-lock braking system. It should be however noted that when the anti-look oontrol is switched to the normal control, the wheel may be locked in the braking operation. This wheel locking causes an abrupt reduction of the vehicle speed and possibly a slip between the wheel and the road surface and deteriorate the driving stability. Specifically, in the four-wheel steering vehicle, the rear wheel is steered from the same phase to the reverse phase as the vehicle speed is reduced from a high speed to a low speed so that the driving stability is seriously affected.